The present invention relates to process control transmitters of the type used in industrial process monitoring and control systems. More specifically, the present invention relates to transmitters which measure process variables in high or low temperature environments.
Process monitoring and control systems are used to monitor and control operation of industrial processes. Industrial processes are used in manufacturing to produce various products such as refined oil, pharmaceuticals, paper, foods, etc. In large scale implementations, these processes must be monitored and controlled in order to operate within the desired parameters.
“Transmitter” has become a term which is used to describe the devices which couple to the process equipment and are used to sense a process variable. Example process variables include pressure, temperature, flow, and others. Frequently, a transmitter is located at a remote location (i.e., in the “field”), and transmits the sensed process variable back to a centrally located control room. (However, field may also include devices positioned locally.) Various techniques are used for transmitting the process variable including both wired and wireless communications. One common wired communication technique uses what is known as a two wire process control loop in which a single pair of wires is used to both carry information as well as provide power to the transmitter. One well established technique for transmitting information is by controlling the current level through the process control loop between 4 mA and 20 mA. The value of the current within the 4 20 mA range can be mapped to corresponding values of the process variable.
One type of transmitter is a pressure transmitter. In general, a pressure transmitter is any type of a transmitter which measures a pressure of a fluid of the process. (The term fluid includes both gas and liquids and their combination.) Pressure transmitters can be used to measure pressures directly including differential, absolute or gauge pressures. Further, using known techniques, pressure transmitters can be used to measure flow or level of the process fluid based upon a pressure differential in the process fluid between two locations.
Typically, a pressure transmitter includes a pressure sensor which couples to the pressure of the process fluid through an isolation system. The isolating system can comprise, for example, an isolation diaphragm which is in physical contact with the process fluid and an isolation fill fluid which extends between the isolation diaphragm and the pressure sensor. The fill fluid preferably comprises a substantially incompressible fluid such as an oil. As the process fluid exerts a pressure on the isolation diaphragm, changes in the applied pressure are conveyed across the diaphragm, through the isolation fluid and to the pressure sensor. Such isolation systems prevent the delicate components of the pressure sensor from being directly exposed to the process fluid.
In some process environments, the process fluid may experience relatively high temperatures. However, transmitters may have a maximum operating temperature of 185° F. Certain transmitters designed for high temperature operation may extend this to 250° F. to 300° F. Temperature extremes can still cause errors in pressure measurements. In processes which have temperatures which exceed the maximum temperature of the pressure transmitter, the transmitter itself must be located remotely from the process fluid and coupled to the process fluid using a capillary tube. The capillary tube can run many feet and an isolation fluid is carried in the tube. One end of the tube mounts to the process through an isolation diaphragm and the other end of the tube couples to the pressure transmitter. This capillary tube and isolation diaphragm is generally referred to as a “remote seal.”
It can be advantageous in some remote seal applications to utilize fill fluids that can further increase the process temperature range with which the transmitter can work. To handle higher process temperatures, fill fluids capable of working at higher temperatures can be used. However, some such fill fluids are either a solid or are extremely viscous at cold ambient temperatures, and may even be so at room temperature.
It is desirable to allow a transmitter to be calibrated at room temperature and/or to be able to transmit and read pressure in a reasonable amount of time at cold ambient conditions before a hot process is brought completely up to temperature. If the fill fluids are solid or too viscous at room temperature, such calibration may not be possible. A heating device can be used to heat the transmitter or remote seal to temperatures at which the high temperature fill fluids have sufficiently low viscosity, but some components of the transmitter or of the remote seal may be damaged at these higher temperatures. For example, even with the use of a high temperature fill fluid, an intermediate fill fluid, with a lower temperature limit, will typically be used to convey process pressures between the high temperature fill fluid and the pressure sensor. This intermediate fill fluid may not be able to withstand heating.